The present invention relates to a method of bonding a plurality of veneer sheets by using water soluble thermosetting adhesive for producing plywood, etc. and an apparatus for carrying out the method.
Heretofore, a plurality of veneer sheets are interposed at desired locations with water soluble thermosetting adhesive and pressed from above and below. At the same time, the thermosetting adhesive is heated and hardened for bonding the veneer sheets. When plywood or composite wood products are manufactured, as shown in Japanese Patent Publication (Kokoku Koho) No. 55(1980)-389998 entitled "Bonding Agent for Plywood using Hop Lees", it is known that such water soluble thermosetting adhesive chiefly comprises, for example, urea resin, melamine resin, phenol resin, or a condensation resin and/or a mixed resin, etc. to which is added a suitable quantity of water, extending agent, hardening agent, etc. according to necessity. Hardening reaction of these resins, for example, urea resin is carried out at a high temperature such as around 100.degree. C. Likewise, phenol resins are hardened at a temperature of around 130.degree. C. In addition, the hardening reaction is made effectively and promptly. Accordingly, the heating temperature of the bonding agent is preferably set to be the boiling point or more of water in its normal state.
On the other hand, there is known a heating type bonding method for bonding veneer sheets by heating the adhesive and hardening thereof with the veneer sheets pressed from below and above. One such example is Japanese Utility Model Application Publication (Kokoku Koho) No. 54(1979)-40545, wherein the so-called hot press method is adapted for pressing stacked veneer sheets from above and below by hot plates. This hot press process is most convenient and widely adopted. However, the conventional multistage or single stage hot press is directly exposed to the air at least at part of the bonding surface of the veneer sheets during the hot pressing operation. Moreover, it has a gas permeability between the air and itself because of existence of vessels within the wooden material. This means that the veneer sheets are substantially in exposed relation with the air.
However, the inventor of the present invention has found out during the course of making the invention the fact that when veneer sheets are bonded by the aforementioned conventional hot press by heating water soluble thermosetting adhesive at a temperature above the boiling point of water in its normal state, hardening of the adhesive is extremely adversely affected.
That is, when the temperature of the adhesive is heated up to or more than the boiling point of water (in the vicinity of 100.degree. C.), moisture contained in the adhesive is boiled and evaporated which causes the adhesive to foam. Moreover, since the bonding surfaces of the veneer sheets are substantially in exposed relation with the air as mentioned in the foregoing, vapor generated normally gradually escapes in the air either directly or through vessels in the veneer sheets. Accordingly, a pressure difference is normally produced between the bonding surfaces and the air. Accordingly, the foaming continues during the hardening reaction of the adhesive. The results are that the adhesive permeates the veneer sheets more than necessary, or is forced out of the bonding surfaces or that the adhesive remaining on the bonding surfaces of the veneer sheets is hardened in caramelized condition. As a result, the resultant product cannot offer an expected sufficient strength.
Of course, it is possible to avoid the foaming of the adhesive by restricting the heating temperature of the bonding agent to a temperature up to or above the boiling point of water in its normal state. However, in this case, the heating is extremely time consuming. In addition some kinds of adhesive are hardened without taking the course by effective reaction. As a result, the resultant product is not practical in use, since the bonding strength thereof is much deteriorated.
In the long run, the conventional means for bonding veneer sheets involves the use of an excessively large quantity of adhesive. Besides, temperature increase of the adhesive is slowed down by latent heat loss, etc. due to vapor leakage. Also, there is a tendency that the heating takes a long time. In addition, the adhesive forced out of the boundary surfaces of veneer sheets and hardened deteriorate quality of the surfaces of the veneer sheets.
The present invention is accomplished in order to overcome the above-mentioned inconveniences inherent in the prior art.